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What can be a reason which OSPF neighbors get stuck in Exstart or Exchange state?

The interface is administratively shutdown

The router cannot see its own RID in the receiving hello packet.

The Router is unable to send unicast updates.

There is a MTU mismatch between OSPF neighbors.

The Router does not receive hello updates.

Explanation

If the router already reached Exstart of Exchange state it means that hello updates have already been exchanged and DR/BDR for broadcast and non-broadcast multi-access networks have been elected. In the states of Exstart and Exchange the process of link state information is exchanged between the routers and their DR/BDR. When two neighbor router stucks in Exstart or Exchange state, is because both routers are from different vendors. Settings such as MTU can result in a mismatch. If a router with a higher MTU sends a packet larger that the MTU set on the neighboring router, the router will ignore the packet. For more information refer to: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13684-12.html